I checked http://nu.nl on an HTC with Android 3.2.4.I do see this character on that page: ë Which ones are missing or which page illustrates the problem?

I can only read your eumlaut on the native browser on my transformer. FF just displays a space instead. Thanks for looking into this!

This is bug https://bugzilla.mozilla.org/show_bug.cgi?id=733124

I think this has been resolved on the UI rewrite you can try the Android build at http://nightly.mozilla.org though the UI for tablets is still a work in progress.

The problem with this is that Android ships some corrupt fonts. As I said above I think we have worked around this on Nightly.